Inhibition of corrosion



United States Patent Office 3,416,974 Patented Dec. 17, 1968 3,416,974 INHIBITION OF CORROSION Robert H. Scott, Corpus Christi, Tex., assignor to Celauesc Corporation, a corporation of Delaware No Drawing. Filed Jan. 25, 1966, Ser. No. 522,816 Claims. (Cl. 148-615) ABSTRACT OF THE DISCLOSURE A process for inhibiting corrosion of metals by liquids contalning acetic acid comprising adding to the liquids a phophorus compound.

The present invention relates to the protection of metals against corrosion. More particularly, it relates to the protection of metals against corrosion by liquids containmg acetic acid. Specifically it relates to inhibition of the corrosive properties of aqueous acetic acid solutions by incorporating certain inhibiting materials thereinto.

Solutions of acetic acid are always corrosive in varying degrees. In addition, crude or partially purified acetic acid solution, such as that which must be handled during the manufacture of acetic acid or in processes utilizing it as a chemical reactant or as a solvent commonly contains other contaminants such as halide ions and formic acid.

While finished acetic acid of commercial purity can be stored and handled Without serious difficulty in containers made of corrosion-resistant materials, corrosion is a serious problem in handling crude acetic acid solutions or in handling even purified acetic acid if at elevated temperatures. In the present art these difliculties are minimized to some extent by employing corrosion resistant alloys, such as stainless steels and higher alloys such as Carpenter 20, Hastelloy F and Incoloy 800, in the fabrication of processing equipment containing liquids such as acetic acid, but even these alloys suffer some deterioration over a period of years, and deterioration can be quite rapid if halide contaminants are present. It is accordingly an object of the present invention to minimize this deterioration of metals which are in contact with liquids comprising a major amount of acetic acid.

It is a further object to provide a method for rendering acetic acid solutions substantially non-corrosive by incorporating into them certain additives which have been found to function effectively as corrosion inhibitors in such liquids.

Other objects of this invention will be apparent from the following detailed description and claims.

In accordance with the present invention, compounds which contain the radical ]:3:O are added to acetic acid solutions comprising from about 3-99 weight percent and preferably from 20-90 weight percent acetic acid and the remainder mostly water in order to prevent or minimize the corrosion of metals which are in contact with said solutions. Substantially any phosphorus compound containing the radical L= may. be employed, including orthophosphoric and pyrophosphoric acid, phosphate salts such as the alkali-metal phosphates including sodium phosphate, sodium hydrogen phosphate, amine phosphates such as the primary, secondary, tertiary and quaternary methyl amine salts, and organic phosphorus compounds such as diphenyl phosphite, triphenyl phosphite and triphenylphosphine oxide. Phosphoric acid is preferred as it is inexpensive, easy to handle, and contains a minimum of inert contaminants. Advantageously, the phosphorus containing additive is fed continuously into the acetic acid solution being treated at a rate suchas to maintain therein a. concentration of the desired magnitude as will be discussed below. If desired, however, the phosphorus compound can be injected intermittently, since the action of the inhibitor has been discovered to be cumulative and, over reasonably short periods of time, comparatively irreversible. That is, the inhibitor forms an adherent coating on the surface of the metal, which tends to persist even when treatment of the liquid is temporarily discontinued.

The corrosion inhibition treatment is effective at very low concentrations. At a concentration of 1000 ppm. of the phosphorus compound, the rate of corrosion in a previously untreated system declines very rapidly to a nominal value within a few hours of the inception of the treatment, but to 1000 ppm. is a convenient and practicable initial treatment rate in a previously untreated system. After an initial period, ranging from a few hours to one or two days depending upon the ratio of exposed metal surface to liquid volume, protection can be maintained at a very low treatment rate, i.e. 1 to 10 p.p.m., so long as there is maintained in the acetic acid solution a definitely detectible quantity of the phosphorus compound. it

Temperature is not a critical factor in the application of the invention. Corrosion inhibitors in general are less effective at high temperatures than at low temperatures, but the invention is effective at temperatures in the elevated range typical of normal distillation operations with acetic acid solutions, i.e. up to about 200 C.

The process of this invention is effective at super-, suband atmospheric pressures.

The invention is effective in the presence of halogencontaining contaminants, both inorganic and organic, at levels ranging from 0 up to about 5000 ppm, calculated as halogen, in the acetic acid solutions. Such contaminants include free halide ions, such as chloride and bromide ions, halogenated acids, such as chloroacetic acid and bromoacetic acid, and halogenated solvents, such as chloroform.

The effectiveness of the invention can be demonstrated, and the degree of protection obtained with a given treatment rate in a given environment can be determined, by utilizing the electrometric methods which are now an established part of the art in corrosion testing and prevention. Briefly, these methods are based upon inserting into the corrosive environment an insulated probe upon which is mounted a conductor, such as a wire, through which a controlled electric current is passed. By measuring the change with time of either (a) the difference in potential required to pass through the wire a constant electric current, or (b) the amount of electric current passing through the wire at a constant potential drop, it is possible to compute the rate of diminution of the cross section of the wire as the metal is corroded away. From this rate of diminution the corrosion rate in inches of metal per year can be computed.

Another means of corrosion testing is the insertion of weighed metallic coupons into a container containing the corrosive liquid, the container kept at conditions substantially duplicating the desired envirdnment. By measuring the weight loss of the coupons after a measured period of time the corrosion rate can be determined.

EXAMPLE I A corrosion test apparatus was prepared comprising (a) a feed tank, (b) a heated corrosion test cell adapted to the insertion of a test specimen in the form of a wire or small diameter tube mounted in an electrical resistancetype corrosion test probe, (c) a cooler, (d) a pressure of Carpenter 20 tubing 3 inches long, 0.0625 inch 0.1).,

and 0.008 inch wall. The approximate composition of Carpenter 20 is 29% nickel, 20% chromium, 2% minimum molybdenum, 3% minimum copper, 1% silicon and 0.07% maximum carbon. The test element was bent into a U shape, the ends of the U being attached to an insulated plug screwed into the wall of the test cell. A solu tion containing approximately 92% acetic acid, 0.5% formic acid, 2000 p.p.m. chloride ion and 5% water was placed in the tank and was pumped through the test cell at a temperature of 127 C. and at a pressure of 100 p.s.i.g. While the acetic acid was being pumped through the test cell, the rate of corrosion of the test element was continuously observed electrometically. The corrosion rate attained a steady state of 0.058 inch per year. Orthophosphoric acid was added to the feed tank in a concentration of 1000 p.p.m. and the pumping of the solution to the test cell was continued. The corrosion rate began to decrease and within four hours it was less than 0.001 inch per year. The cell had a volume of 500 milliliters and the flow rate was 500 milliliters per hour.

EXAMPLE II A second sample of acetic acid solution having approxi mately the same composition as that described in Example I was placed in the feed tank and the solution fed through the test cell at a temperature of 132 C. and at a pressure of about 100 p.s.i.g. The Carpenter test element had a corrosion rate of 0.657 inch per year in this solution. Pyrophosphoric acid was added to the acetic acid solution in the feed tank to a concentration of 1000 p.p.m. In about three and one-half hours the corrosion rate dropped to less than 0.001 inch per year. The probe did not reactivate in an additional twenty-four hours.

EXAMPLE III An acetic acid solution with the composition similar of that described in Example I was pumped through a test cell at a temperature of 130 C. and at a pressure of 100 p.s.i.g. The test element was of Carpenter 20. The corrosion of this Carpenter 20 probe at 130 C. was 0.027 inch per year. The probe was passivated by adding pyrophosphoric acid in a concentration of 1000 p.p.m. The corrosion rate decreased to less than 0.001 inch per year in approximately fourteen hours. The inhibitor concentration was then decreased to about 300 p.p.m. Although the temperature was increased to 139 C. there was no corrosion with 300 p.p.m. of the inhibitor.

EXAMPLE 1v An acetic acid solution having the approximate composition, 90% acetic, 2% wate-r, 6% acetal'dehyde, and 0.5% formic acid, was fed through the test cell at a temperature of about 135 C. and at a pressure of 100 p.s.i.g. The corrosion rate was determined electrometrically and found to be 0.075 inch per year. Pyrophosphoric was added to the feed tank sample of the above acetic acid composition to give a concentration of 1000 p.p.m. The corrosion rate began to decrease and after approximately two hours it was less than 0.001 inch per year.

EXAMPLE V Air was removed from a solution of 79 weight percent reagent grade acetic .acid and 21 weight percent water by saturating the solution with nitrogen. Coupons of Type 316 stainless steel were treated electrolytically to remove the protective oxide film and these coupons were then carefully weighed. After weighing, the coupons were placed in a test yell and the cell half filled with the acetic acid-Water solution. The test cell was closed and placed in a heater and held at a temperature ranging from 300 C. to 310 C. and at its vapor pressure for approximately forty-seven hours. After this period of time the container was cooled to ambient temperature, the coupons removed, cleaned and weighed. Corrosion rates calculated from weight loss were 0.024 and 0.032 inch per year. Orthophosphoric acid in a concentration of 1000 p.p.m. was then added to the liquid in the test cell and the above procedure carried out again for a period of fortyeight hours. After this time the corrosion rate was measured at 0.0005 inch per year.

EXAMPLE VI Ten weight percent fiber grade terephthalic acid was added to a solvent containing 19.6 weight percent water and 80.4 Weight percent acetic acid. Sutficient triethylphosphite to give a concentration of 0.1 weight percent was added to the solution and the solution was placed in the test cell. Type 316 stainless steel coupons were suspended in the liquid. A vacuum pump was used to remove air from the vapor space above the liquid and the container was pressure to 0 p.s.i.g. with nitrogen. The test cell was placed in a heater and brought: to a temperature ranging from 285 C. to 300 C. for a period of fortyeight hours. The coupons were removed, cleaned and weighed. There was no measurable corrosion in the liquid phase.

EXAMPLE VII A one liter high pressure container was charged with 500 milliliters of a solution containing 78.4 weight per cent of acetic acid and 21.6 weight percent water. Type 316 stainless steel coupons were suspended in the liquid. Triphenylphosphine oxide in a concentration of 1000 p.p.m. was added to the solvent, the test container closed, evacuated with a vacuum pump, purged with nitrogen, and placed in a heater. The container was held at 300 C. for forty-eight hours. The coupons were removed, cleaned and weighed. The corrosion rate was 0.0008 inch per year in the liquid phase,

EXAMPLE VIII A one liter high pressure container was charged with 500 milliliters of a solution containing weight percent acetic acid and 20 weight percent water. Triphenylphosphine was added in a concentration of 1000 p.p.m. to the solution. Type 316 stainless steel coupons were suspended in the liquid, the container closed, dissolved air removed by evacuating the vapor space with a vacuum pump and the vapor space filled with nitrogen. The test container was placed a heater and heated to a tem;

The embodiments of the invention in which an exclusive property or-privilege is claimed are defined as follows:

1. A method for protecting a metal against corrosion by a solution comprising 20 to 99 weight percent of acetic acid, the solution being in contact with the metal, which method comprises incorporating a phosphorus compound having the radical U into the solution, the amount of the phosphorus compound being sufficient to substantially reduce the rate of corrosion of the metal by the solution.

2. The method of claim 1 wherein the phosphorus compound is one selected from the group consisting of pyrophosphoric acid, orthophosphoric acid, alkali metal phosphates, primary, secondary, tertiary and quaternary methyl amine salts of phosphoric acid diphenylphosphite, triphenylphosphite, and triphenylphosphine oxide.

3. The method of claim 1 wherein the phosphorus compound is present in a concentration ranging from 1 to 1000 p.p.m.

4. The method of claim 1 wherein the metal is one selected from the group of stainless steels and a metal having a composition of about 29% nickel, 20% chromium, 2% minimum molybdenum, 3% minimum copper, 1% silicon, and 0.07% maximium carbon.

5. The method of claim 1 wherein the solution comprises water, acetic acid and a halide contaminant present in a concentration ranging from 1 to 5000 ppm. calculated as halide.

6. A method for protecting a metal against corrosion by a solution comprising 20 to 90 weight percent of acetic acid and the remainder comprising water and halide contaminant, the solution being in contact with the metal, which method comprises initially incorporating 100 to 1000 ppm. of a phosphorus compound containing the radical l into the solution to form a protective coating on the surface and thereafter maintaining the protective coating on the surface,

7. The method of claim 6 wherein the phosphorus compound is one selected from the group consisting of orthophosphoric acid, pyrophosphoric acid, primary, secondary, tertiary and quaternary methyl amine salts of phosphoric acid, alkali metal phosphates, diphenylphosphite, triphenylphosphite and triphenylphosphine oxide.

8. The method of claim 6 wherein the metal is one selected from the group consisting of stainless steels and a metal having a composition of about 29% nickel, 20% chromium, 2% minimum molybdenum, 3% minimum copper, 1% silicon, and 0.07% maximum carbon.

9. The method of claim 1 wherein the phosphorus compound is phosphoric acid.

10. The process of claim 8 wherein the phosphorus compound is phosphoric acid.

References Cited UNITED STATES PATENTS 2,416,734 3/1947 Boggs et al.

2,471,907 5/1949 Snyder 148-6.15 2,789,070 4/1957 Copelin 2- 148-615 2,873,196 2/1959 Baevsky 10614 2,901,438 8/1959 Rogers 212.7 XR 3,007,780 11/1961 Beigay et al 252-79.3 XR 3,024,148 3/1962 Schaer 25279.3 XR

JULIUS FROME, Primary Examiner.

L. B. HAYES, Assistant Examiner.

U.S. Cl. X.R. 

